1. Field of the Invention
The present invention relates to high purity sulfonic acids and their use in electrochemical processes, methods for preparing such high purity acids, methods for preparing high purity metal-sulfonate or sulfonic acid solutions, and products formed by using such methods and solutions.
2. Prior Art
Electrochemical processes may contain acid electrolytes to impart conductivity and thus lower the required voltage, aid in the dissolution of metal (e.g., formation of metal salts) and metal oxides (e.g., descaling), aid in the deposition of metals from aqueous solutions (e.g., electrodeposition), and may be used in the synthesis of conductive polymers to provide proton conduction and in batteries to aid in the dissolution and solvation of metals such as zinc and lead.
There are many acids that may be used in the above applications such as sulfuric, nitric, phosphoric, sulfamic, and hydrochloric acid as well as phosphonic and sulfonic acids. The choice of the acids is very dependent upon the application and the purity of the acid. For example, hydrochloric acid is not typically used to descale ferrous-based metals due to its propensity for pitting corrosion in the metal. Sulfuric acid is not often used in the electrodeposition of silver or lead-based alloys due to the likelihood of metal precipitation in this acid.
There are also usually several grades or purities of acids and the choice of grade is often again dependent upon the industrial application. For example, sulfuric acid may be purchased as technical grade, reagent grade, electronic grade and microelectronic grade. Technical grade has the most impurities and is therefore not used in metal electrodeposition solutions as required in the microelectronic field.
Recently, sulfonic acids, including alkane sulfonic acids, have gained acceptance in many commercial applications due in large part to (a) their ability to solubilize metals that are insoluble in acids such as sulfuric acid, are typically non-corroding acids like hydrochloric, (b) being reducing acids unlike nitric acid and (c) being more stable than sulfamic acid at low to moderate pH and elevated temperatures.
Additionally, improvements in electroplating solutions and techniques have been made (a) to meet increased standards for plating and (b) to be able to plate under more highly difficult circumstances.
Most improvements focused on the use of organic additives such as suppressor additives, accelerator additives and leveling additives and the like to obtain the desired results.
However, even with the improvements in electroplating processes, circumstances may exist that can lead to plating defects due to inadequate coverage in recess areas such as in the vias or trenches or through-holes in electronic devices, poor corrosion resistance of the deposited metal, too high a residual stress leading to cracking in the metal coating or a rough, commercially unacceptable deposit. These defects can occur as a result of an imbalance of the organic additives intended to obtain the desired metallic coating.
Also, the useful lifetime of many metal electrodeposition electrolytes is dependent upon the breakdown of the organic additives, particularly the sulfur-containing accelerator additive.
It is well known in the industry that by having typical low valent sulfur-containing accelerator type additive such as those used in acid copper, nickel, cobalt and iron solutions, uniform plating of particularly low to high aspect ratio vias and microvias and other difficult-to-plate electronic features such as through-holes in printed circuit boards is possible.
Typical accelerator or brightener additives contain one or more low valent sulfur atoms, and typically without any nitrogen atoms and a molecular weight of about 1500 or less. In all cases, the low-valent sulfur accelerator or brightener decomposes to impart the desired effects.
However, combinations of various low-valent sulfur compounds is typically unwanted due to competitive interactions at the surface of the work piece.
Therefore, it would be desirable to control the concentration of the wanted low-valent sulfur additive in narrow ranges, often in the milligram per liter range, and to avoid the interaction with unwanted low-valent sulfur impurity molecules from the acid make-up solution.
The synthesis of conductive polymers such as polyaniline may also employ sulfonic acids to impart conductivity through protonic doping. Choi and co-workers in Synthetic Metals showed aniline polymerization in the presence of dodecylbenzesulfonic acid exhibited good electrical conductivity. Dominis and co-workers in Synthetic Metals, studied the synthesis of polyaniline in a solution containing nonylnaphthalene sulfonic acid and found that the sulfonic acid aids in the solvation of the conductive polymer in organic media. There was no mention of the purity of the sulfonic acid used in these studies.
During the electrodeposition of nickel, iron and cobalt, additives may be employed in the aqueous electroplating solution to impart brightness to the metal deposit, decrease the stress in the metal coating, increase the corrosion protection of the underlying substrate or to achieve a desired esthetic appearance.
The composition of the additives used in the electroplating solution is dependent upon the desired metal coating. However, in general, additives for these metal electroplating baths may contain sulfur moieties as described by Lowenheim in Modern Electroplating, 3rd Ed. Class 1 brighteners reduce the grain size of the metal coating but also incorporate a small, about 0.03%, amount of sulfur. In the corrosion protection of steel, a duplex nickel coating is required whereby the underlying nickel (e.g., close to the steel) must not contain any sulfur in the deposit. The nickel electrolyte is formulated such that the additives do not contain reducible sulfur compounds that may eventually be co-deposited in the nickel. The top nickel layer of the duplex contains small amounts of sulfur and corrodes preferentially compared to the underlying sulfur-free nickel coating. Bodnevas and Zahavi in Plating and Surface Finishing, (December 1994, pg. 75) showed the effects of sulfur-bearing additives on the internal stress of nickel deposit, the incorporation of sulfur into the nickel coating and the relationship between the sulfur-additive concentration in the solution and the incorporation of elemental sulfur in the nickel deposit plated from sulfate-based solutions. If the nickel electrolytes are made using impure sulfonic acids such as those containing reduced or easily reducible sulfur compounds, the likelihood of sulfur incorporation increases significantly thus affecting the resultant stress in the deposit, the brightness of the metallic coating and the corrosion properties.
In depositing low stress nickel coatings, for use in aerospace applications, from sulfamate-based solutions, no sulfur is co-deposited in the coating even if using a conventional sulfur-bearing stress reducing agent such as 1,3,5, naphthalenetrisulfonic acid (NTS). In NTS, sulfur has an oxidation state of +6 and is not easily reduced. However, reducible sulfur compounds, if present in impure sulfonic acids, may alter the grain size of the nickel deposit and consequentially alter the stress in the metallic nickel deposit.
The synthesis of sulfonic acids may be complex and several undesirable impurities may be present in the desired sulfonic acids leading to difficulties in using sulfonic acids in electrochemical processes. Sulfonic acids may be made via the oxidation of the corresponding thiol, by hydrolysis of alkanesulfonyl halide, or by the oxidation of dimethyldisulfide. Various impurities may also be made during the oxidation or hydrolysis reaction and thus must be removed prior to use. Many low valent sulfur compounds such as sulfur (II) or sulfur (IV) or higher valent sulfur molecules such as sulfur (VI) compounds that are susceptible to reduction and are present in the sulfonic acid may produce a stench or odor, interfere with the ongoing electrochemical process or alter the final product.
Low valent or easily reducible sulfur compounds in an acidic medium may also produce an undesired sulfur stench. This stench comes from the formation of minute amounts of hydrogen sulfide, dimethylsulfide or sulfur dioxide. These materials are unwanted and dangerous during electrochemical processes.
It thus would be desirable to have new electrochemical compositions based on high purity sulfonic acids. It would be particularly desirable to have new sulfonic acid compositions that can be effectively used with metals of strong reducing capabilities such as tin, zinc and iron without deleterious effects such as odor and defects in the metal or polymer deposit. Such compositions could be used in electrodeposition, batteries, conductive polymers and de-scaling applications.